Noise Mitigation in Thermocouple Signal Conditioning System for Neutral Beam Calorimeter for NBI SST-1

This paper presents the noise mitigation techniques employed for thermocouple signal conditioning system for a neutral beam injection system. Thermocouple instrumented beam calorimeter system is used to characterize the beam by determining the beam parameters like power profile, divergence, neutralization efficiency, and so on. Signal conditioning system for beam calorimeter requires significant attention toward electromagnetic interference as thermocouple signals are very sensitive and beam extraction environment is substantially noisy due to fast application of high voltage (HV) for beam extraction. Due to common occurrence of breakdowns in between the grids of the accelerator, the HV power supply is required to turn OFF in few microseconds. Reapplication of HV too is required in such cases. The large dV/dt associated with turn ON and turn OFF of HV causes significant amount of current in ground conductor. Even though thermocouples are mounted on components at ground potential, these too are subjected to voltage transients and they pickup noise in turn. A signal conditioning system, without proper attention to noise reduction techniques cannot reproduce noise free information. A signal conditioning and data acquisition (DAQ) system is developed to measure the voltage developed across the thermocouples due to rise in temperature. This signal conditioning and DAQ system is capable of capturing the temperature rise information in the presence of voltage transients associated with HV switching during breakdowns. This paper highlights the techniques used for noise mitigation in signal conditioning system and obtained results.